DS1921 datasheet(14/44 Pages) DALLAS

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DS1921H/Z

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and periods (12 periods for too hot and 12 for too cold). The date and time of each of these periods can be

determined from the Mission Time Stamp and the time distance between each temperature reading.

ALARM TIME STAMPS AND DURATIONS ADDRESS MAP Figure 8

ADDRESS

DESCRIPTION

ALARM EVENT

0220h

Mission Samples Counter Low Byte

0221h

Mission Samples Counter Center Byte

Low Alarm 1

0222h

Mission Samples Counter High Byte

0223h

Alarm Duration Counter

0224h to 0227h

Alarm Time Stamp and Duration

Low Alarm 2

0228h to 024Fh

Alarm Time Stamp and Durations

Low Alarms 3 to 12

0250h

Mission Samples Counter Low Byte

0251h

Mission Samples Counter Center Byte

High Alarm 1

0252h

Mission Samples Counter High Byte

0253h

Alarm Duration Counter

0254h to 0257h

Alarm Time Stamp and Duration

High Alarm 2

0258h to 027Fh

Alarm Time Stamp and Durations

High Alarms 3 to 12

The alarm time stamp is a copy of the Mission Samples Counter when the alarm first occurred. The least

significant byte is stored at the lower address. One address higher than the time stamp the DS1921H/Z

maintains a 1-byte duration counter that stores the number of samples the temperature was found to be

beyond the threshold. If this counter has reached its limit after 255 consecutive temperature readings and

the temperature has not yet returned to within the tolerance band, the device will issue another time stamp

at the next higher alarm location and open another counter to record the duration. If the temperature

returns to normal before the counter has reached its limit, the duration counter of the particular time

stamp will not increment any further. Should the temperature again cross this threshold, it will be

recorded at the next available alarm location. This algorithm is implemented for the low as well as for the

high temperature threshold.

MISSIONING

The typical task of the DS1921H/Z is recording the temperature of a temperature-sensitive object. Before

the device can perform this function, it needs to be configured. This procedure is called missioning.

First of all, DS1921H/Z needs to have its RTC set to valid time and date. This reference time may be

UTC (also called GMT, Greenwich Mean Time) or any other time standard that was chosen for the

application. The clock must be running (EOSC = 0) for at least one second. Setting a RTC alarm is

optional. The memory assigned to storing alarm time stamps and durations, temperature histogram, as

well as the Mission Time Stamp, Mission Samples Counter, Mission Start Delay, and Sample Rate must

be cleared using the Memory Clear command. In case there were temperature alarms in the previous

mission, the TLF and THF flags need to be cleared manually. To enable the device for a mission, the EM

flag must be set to 0. These are general settings that have to be made regardless of the type of object to be

monitored and the duration of the mission.

Next, the low temperature and high temperature thresholds to specify the temperature tolerance band

must be defined. How to convert a temperature value into the binary code to be written to the threshold

registers is described under Temperature Conversion earlier in this document.

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